Memory leak in png_read_destroy function.

From: Timothy <kysantt@gmail.com>
Date: Mon, 15 Jul 2013 09:04:17 +0900

png file destroy sequence : BitmapRegionDecoder recycle > nativeClean >
~SkPNGImageDecoder > ~SkPNGImageIndex > png_destroy_read_struct > png_read_destroy

Cause memory leak after calling png_read_destory.
png_index_struct could NOT be on free logic because png_ptr->index is NULL
after png_memset(png_ptr, 0, png_sizeof(png_struct));

Change-Id: I21315b01b46648b4dca27a487dc88ace7a065e95